A string is stretched by two equal but opposite forces f newton each what is tension in string

There is actually no force of tension in the string as the strings holds two forces acting on the same direction. It is zero.

Explanation:

If there was a tension in the string the two opposing forces with two equal magnitude will be pulling in the opposite directions.

When the f newton of the right is pulled one side and the f newton of the left is pulled the other side, there would be only one action by the force. Since there is one action in the same string in the opposite directions the tension can only be the magnitude of the string.

Indeed, there is actually no force of tension in the string as the strings holds two forces acting on the same direction. It is zero.